Belgian Pat. No. 791,161 discloses inter alia antibacterial agents of the formula (II): ##SPC2##
wherein R is an acyl group as found in known antibacterially active penicillins and cephalosporins. It has now been discovered that if the 3-benzyl group of the compounds of formula (II) is replaced by a benzyl group substituted by a carboxylic acid group then many of the resulting compounds have a surprisingly prolonged serum half-life.